System and methods for memory expansion

ABSTRACT

This document discusses, among other things, an example system and methods for memory expansion. An example embodiment includes detecting a memory command directed to a logical rand and a number of physical ranks mapped to the logical rank. The example embodiment may also include issuing the memory command to the number of physical ranks based on determining that the memory command is to be issued to the number of physical ranks.

TECHNICAL FIELD

This patent document pertains generally to computer architecture andmore particularly, but not by way of limitation, to a system and methodfor memory expansion.

BACKGROUND

Typical main memory systems consist of one or more memory channels andone or more memory modules connected to each channel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.Like numerals having different letter suffixes represent differentinstances of substantially similar components. The drawings illustrategenerally, by way of example, but not by way of limitation, variousembodiments discussed in the present document.

FIG. 1 is a block diagram illustrating a computer system, in accordancewith example embodiments;

FIG. 2 is a block diagram of a memory system coupled to a processor, inaccordance with an example embodiment;

FIG. 3 is a flow diagram illustrating a method for memory expansion, inaccordance with an example embodiment;

FIG. 4 is a block diagram of a memory system, in accordance with anexample embodiment;

FIG. 5 is a flow diagram illustrating a method for programming a memorycontroller, in accordance with an example embodiment;

FIG. 6 is a chart showing a memory expansion map, in accordance with anexample embodiment;

FIG. 7 is a block diagram showing an example relationship betweenlogical Registered Dual Inline Memory Modules (RDIMMs) and the physicalRDIMMs;

FIG. 8 is a flow diagram of a method for storing location information,in accordance with an example embodiment;

FIG. 9 is a flow diagram illustrating a method for reporting a memoryerror, in accordance with an example embodiment.

FIG. 10 is a block diagram of a memory expansion system, in accordancewith an example embodiment;

FIG. 11 is a flow diagram of a further method for memory expansion, inaccordance with an example embodiment.

FIG. 12 is a block diagram showing example address and control pathcircuitry, in accordance with an example embodiment;

FIG. 13 is a flow diagram of a method for distributing a memory command,in accordance with an example embodiment;

FIG. 14 is a flow diagram of illustrating a method for modifying aconfiguration value, in accordance with an example embodiment;

FIG. 15 is a block diagram illustrating example address and control pathcircuitry and data connection circuitry, in accordance with an exampleembodiment; and

FIG. 16 is a flow diagram illustrating a method for controlling a dataflow, in accordance with an example embodiment.

DETAILED DESCRIPTION Overview

In general, an example system and methods are described for memoryexpansion.

In an example embodiment, a logical memory location corresponds to alogical rank on an RDIMM. The RDIMM may include four logical ranks ofdynamic random access memory (DRAM) devices that each have 4 Gigabit(Gb) capacity and are four bits wide. An example mapping module of thepresent disclosure may receive an activate command from a memorycontroller to activate a row of memory within one of the logical ranks.

In an example configuration, physical ranks rather than logical ranksare actually coupled to the memory controller. Thirty-two physical ranksof memory devices (e.g., on eight RDIMMs), each having DRAM devices with1 Gb capacity and being four bits wide, may be physically coupled to thememory controller via the mapping module. In response to the activatecommand, the example mapping module may map a logical rank number and aportion of the associated row address to one of the thirty-two physicalranks (and e.g., a sub-channel) to which a memory access requestcorresponds.

The memory controller may be configured by a basic input output system(BIOS) module to operate with the mapping module during initializationof the memory system. Configuration of the memory controller may includeconverting serial presence detect (SPD) information from the physicalRDIMM to SPD information from a logical RDIMM.

Once configured, the memory controller may attempt to read from or writeto a memory cell in an activated row of memory within a logical rank.Some example embodiments further include storing, in response to anactivate command, the physical rank information associated with thecommand for each bank in a logical rank. A logical rank identification(ID) and a bank ID appearing in the initial activate command and thesubsequent read, write or other command, etc., may be used to locate thephysical rank corresponding to the activated row.

The mapping module may include a map and control unit and a data pathunit. An example map and control unit may be configured to map logicalmemory addresses to physical memory addresses and to manage memorycommands. Example data path units may control the flow and timing ofdata flow between the memory controller and physical memory devices. Toconnect a memory controller to a mapped physical rank, the example datapath unit may connect a single communication channel to any one ofmultiple sub-channels using field effect transistors (FETs) as a switch.

In example embodiments, some memory commands issued to a logical rank bythe memory controller are to be distributed by the mapping module to thephysical ranks mapped to the logical rank. Mode register write commandmay also be modified to account for latency differences between logicaland physical memory devices.

A memory controller may detect an error on a logical RDIMM using anerror correction code (ECC). Some embodiments may include a BIOS modulethat may determine a physical memory address corresponding to a logicalmemory address reported by the memory controller as being the source ofa memory error.

This overview is intended to provide an overview of the subject matterof the present patent application. It is not intended to provide anexclusive or exhaustive explanation of what is claimed. The detaileddescription is included to provide further information about the subjectmatter of the present patent application.

Example Embodiments

The following detailed description includes references to theaccompanying drawings, which form a part of the detailed description.The drawings show illustrations in accordance with example embodiments.These embodiments, which are also referred to herein as “examples,” aredescribed in enough detail to provide for those skilled in the art topractice the disclosure. The embodiments may be combined, otherembodiments may be utilized, or structural, logical and electricalchanges may be made without departing from the scope of what is claimed.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope is defined by the appended claims andtheir equivalents.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one. In this document, the term“or” is used to refer to a nonexclusive or, such that “A or B” includes“A but not B,” “B but not A,” and “A and B,” unless otherwise indicated.Furthermore, all publications, patents, and patent documents referred toin this document are incorporated by reference herein in their entirety,as though individually incorporated by reference. In the event ofinconsistent usages between this document and those documents soincorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

FIG. 1 is a block diagram illustrating a computer system 100 inaccordance with example embodiments. Within the computer system 100 is aset of instructions for causing the machine to perform any one or moreof the methodologies discussed herein. In alternative exampleembodiments, the machine operates as a standalone device or may beconnected (e.g., networked) to other machines. In a networkeddeployment, the machine may operate in the capacity of a server or aclient machine in a server-client network environment, or as a peermachine in a peer-to-peer (or distributed) network environment. Themachine may be a personal computer (PC), a tablet PC, a set-top box(STB), a Personal Digital Assistant (PDA), a cellular telephone, a webappliance, a network router, switch or bridge, or any machine capable ofexecuting a set of instructions (sequential or otherwise) that specifyactions to be taken by that machine. Further, while only a singlemachine is illustrated, the term “machine” shall also be taken toinclude any collection of machines that individually or jointly executea set (or multiple sets) of instructions to perform any one or more ofthe methodologies discussed herein.

The example computer system 100 includes a processor 102 (e.g., acentral processing unit (CPU), a graphics processing unit (GPU) orboth), a memory system 104 and a static memory 106, which communicatewith each other via a bus 108. The main memory system 104 may include amemory controller, a memory expansion system and a volatile memory, each(not shown) discussed in more detail below. The static or non-volatilememory 106 may store BIOS to act as an interface between system hardwareand system software. The BIOS may initialize system hardware uponboot-up of the computer system.

The computer system 100 may further include a video display unit 110(e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)). Thecomputer system 100 also includes an alphanumeric input device 112(e.g., a keyboard), a user interface (UI) navigation device 114 (e.g., amouse), a disk drive unit 116, a signal generation device 118 (e.g., aspeaker) and a network interface device 120.

The disk drive unit 116 includes a machine-readable medium 122 on whichis stored one or more sets of instructions 124 and data structures(e.g., software) embodying or utilized by any one or more of themethodologies or functions described herein. The instructions 124 mayalso reside, completely or at least partially, within the main memorysystem 104 and/or within the processor 102 during execution thereof bythe computer system 100, the main memory system 104 and the processor102 also constituting machine-readable media.

The instructions 124 may further be transmitted or received over anetwork 126 via the network interface device 120 utilizing any one of anumber of well-known transfer protocols (e.g., file transfer protocol(FTP)).

While the machine-readable medium 122 is shown in an example embodimentto be a single medium, the term “machine-readable medium” should betaken to include a single medium or multiple media (e.g., a centralizedor distributed database, and/or associated caches and servers) thatstore the one or more sets of instructions. The term “machine-readablemedium” shall also be taken to include any medium that is capable ofstoring, encoding or carrying a set of instructions for execution by themachine and that cause the machine to perform any one or more of themethodologies of the present disclosure, or that is capable of storing,encoding or carrying data structures utilized by or associated with sucha set of instructions. The term “machine-readable medium” shallaccordingly be taken to include, but not be limited to, solid-statememories, optical and magnetic media.

FIG. 2 is a block diagram of a memory system 204 coupled to a processor224, in accordance with an example embodiment. The memory system 204 isshown to include a memory controller 222 to provide a memory address202, a mapping module 206 and memory module B 210. FIG. 2 shows anexample relationship between the memory address 202 and memory cells XYZ220 of the logical memory module A 218 and an example relationshipbetween the memory address 202 and the memory cells 212 of the physicalmemory module B 210. The memory system 204 may be substantially similarto the memory system 104 in FIG. 1.

The processor 224 may access instructions that are stored within thememory cells 212 and 214 and may process the instructions in connectionwith the operation of the computer system 100 of FIG. 1. The memorycells 212 and 214 may store instructions or any other software code,records or any other data accessed by the processor 224, or byapplications or operating systems (not shown) running on the computersystem 100.

The processor 224 may be electronic circuitry, for example, electroniccircuitry formed within a semi-conductor chip. However the processor 224is not limited to being electronic circuitry and may perform thefunction of executing instructions in any fashion known by one havingordinary skill in the art. The processor 224 may include an interface225 to transmit information to and receive information from the memorycontroller 222 over the communication channel 226. The memory controller222 may include an interface 221 for transmitting information to andreceiving information from the processor 224 via a communication channel226.

The memory controller 222 may receive requests from the processor 224 toaccess the memory on memory module B 210. Communications between theprocessor 224 and the memory controller 222 may be made using acommunication protocol (e.g., front side bus protocol, etc). In anexample embodiment, the memory controller 222 may be integrated with theprocessor 224. An example memory controller 222 may receive a memoryaccess request from the processor 224 by way of one communicationprotocol (e.g., a front side bus protocol) but may use a differentcommunication protocol (e.g., double data rate (DDR)) to communicate therequest to the memory module B 210 over the communication channel 216(e.g., a computer bus). In an example embodiment, the memory controller222 may translate the protocol used by the processor 224 into a protocolused to forward the request to the memory module B 210 via thecommunication channel 216.

The memory controller 222 may control the locations in which certaininformation is stored within the memory cells 212, 214 of the memorymodule B 210. In an example embodiment, the memory controller 222 mayperform various other operations associated with maintaining the storageof information within the memory cells 212, 214 of the memory module B210 (e.g., refreshing memory cells or any other maintenance operation,etc.). The functionality performed by the memory controller 222 may beimplemented through logical operations. Such logical operations may beemployed with software, hardware or a combination of software andhardware.

The memory address 202 may be sent by the memory controller 222 to thememory module B 210 to identify memory cells 212, 214 that are to beaccessed. The memory controller 222 may transmit the memory address 202over the communication channel 216 via the interface 223. In an exampleembodiment, the memory cells are addressed by the memory address 202using the binary numeral system by encoding a sequence of zeros and onesinto a message indicating a location. However, any coding scheme may beused to encode the location of a memory cell or memory cells within amemory system. In an example embodiment, the memory address 202 encodesan address for both the memory cells XYZ 220 of the logical memorymodule A 218 and the memory cells 212 of the physical memory module B210.

In example embodiment, the memory controller 222 transmits the memoryaddress 202 as if the address were bound for the memory cells XYZ 220 ofthe memory module A 218. In this embodiment, the memory controller 222expects to access the memory cells XYZ 220 on the memory module A 218.In an example embodiment, the memory system 204 does not include thememory cells XYZ 220 of the memory module A 218 and the memory system204 does include the memory module B 210 including the memory cells 212and the memory cells 214. The mapping module 206 may receive the memoryaddress 202 at its interface 207 and map the memory address 202 to thememory cells 212 of the memory module B 210.

The mapping module 206 may include a map 208. The mapping module 206 mayreference the map 208 to identify the memory cells 212 from the memoryaddress 202. In an example embodiment, the mapping module 206 mayreceive the memory address 202 and may use the memory address 202 or aportion of the memory address 202 as an index in the map 208 for thepurpose of identifying a different address (not shown) associated withthe memory address 202 or the portion of the memory address 202. The newaddress (not shown) may correspond to the memory address 202 and mayidentify the address of the memory cells 212. In an example embodiment,the map 208 is a data structure (e.g., a look-up table (LUT), an index,a linked list or any other data structure appropriate to implement themap 208).

An example mapping module 206 may include an interface 207 to receivethe memory address 202 and an interface 209 to access the contents ofthe memory module B 210 over the communication channel 217. It is to beappreciated that the communication channel 217 may include multiplesub-channels over which data may travel. A logical structure may formthe functionality provided by the mapping module 206. The logicalstructure may be implemented using software, hardware or a combinationof software and hardware.

As previously described, the memory module B 210 may include the memorycells 212 and 214, which may each include a set of memory cells. A setof memory cells may include one or more memory cells. In an exampleembodiment, the memory module B 210 is a dual inline memory module(DIMM) that is configured to host multiple memory devices. The sets ofmemory cells 212 and 214 may reside within multiple memory devices wherethe individual memory cells may be located. In an example embodiment,each memory device is a DRAM. A DRAM is a semiconductor memory devicethat stores information in memory cells constructed with capacitors. Inan example embodiment, each set of memory cells 212, 214 is a “rank” ofmemory devices (discussed in further detail below). Further, each rankof memory devices may include one or more “banks” of memory cells(discussed in further detail below) and each bank of memory cells mayinclude an array of rows and columns, the intersection of which are thelocation and the address of a memory cell.

The memory module B 210 may include an interface 211 to accommodateaccess requests for the memory cells 212 and the memory cells 214 or anyindividual memory cell within the memory cells 212 and 214. In anexample embodiment, various types of memory accesses may includeactivating a row of cells, reading or writing a memory cell,pre-charging a row of memory cells, refreshing a memory cell or anyother memory access operation known in the art.

Various constraints (e.g., standard body specifications, physicalproperties, etc.) may limit the number of physical ranks directlyattached to the memory controller. The memory expansion system andmethod described herein may be used within the bounds of suchconstraints.

FIG. 3 is a flow diagram illustrating a method 300 for memory expansion,in accordance with an example embodiment. At block 302 the method 300includes receiving a memory address over a communication channel. Thememory address may identify a location of a first set of memory cells.Referring to FIG. 2, in an example embodiment, the mapping module 206includes an interface 207 to receive the memory address 202 over thecommunication channel 216 (e.g. a first communication channel). Thememory address 202 may identify a location of the memory cells XYZ 220of the memory module A 218 (e.g., a first set of memory cells).

At block 304, the method 300 includes mapping the memory address to asecond location of a second set of memory cells. The second set ofmemory cells may be one of multiple sets of memory cells and may bedifferent or other than the first set of memory cells. As describedabove, the mapping module 206 may be coupled to the memory module B 210.The memory module B 210 is shown to include multiple sets of memorycells 212 and 214. In an example embodiment, the memory module B 210does not include the memory cells XYZ 220 (e.g., the first set of memorycells). The mapping module 206 may map the memory address 202 to thelocation of memory cells 212 within the memory module B 210.

The map 208 may associate multiple memory addresses with multiple setsof memory cells. For example, the mapping module 206 may reference themap 208 to associate one of the multiple memory addresses (e.g., thememory address 202) with one of the multiple sets of memory cells suchas the sets of memory cells 212 and 214 of the memory module B 210.

In an example embodiment, location information related to the locationof the second set of memory cells (e.g., memory cells 212) may be storedwithin the mapping module 206, the larger memory system 204 or anystorage accessible by the mapping module 206. Upon receiving asubsequent request (e.g., a read request from the memory controller 222)to access the second set of memory cells, the location information maybe accessed and used to locate the second set of memory cells (e.g., thememory cells 212). Storing location information is discussed in furtherdetail below.

FIG. 4 is a block diagram of a memory system 400, in accordance with anexample embodiment. The memory system 400 is shown to include a memorycontroller 422 coupled to a mapping module 406 over a communicationchannel 416 that is further coupled to eight physical (e.g., actual)RDIMMs 430-437 over sub-channels 425-428. It is to be appreciated thatspecific implementations may employ fewer or greater numbers of physicalRDIMMs. An RDIMM registers address and control signals before forwardingthe signals to the DRAMs residing on the RDIMMs.

A BIOS module 436 may be coupled between the memory controller 422, themapping module 406 and the physical RDIMMs 430-437 via the communicationbus 434. The BIOS module 436 is shown to include a map module 440 and aconversion module 438. In an example embodiment, the BIOS module 436 isto program the memory controller during system initialization using theconversion module 438 and based on information collected from thephysical RDIMMs 430-437. The manner in which the conversion module 438is used by the BIOS module 436 to program the memory controller isdiscussed in further detail below.

When the RDIMMs are in use, the BIOS module 436 may receive from thememory controller 422 via the communication buses 434, 435, the logicaladdress of an error that has occurred on the physical RDIMMs 430-437.The memory controller 422 may find errors in memory data by detectingunexpected values in an ECC embedded within packages of retrieved memorydata. The map module 440 may use logical address information (and e.g.,other information associated with the error) to derive the physicaladdress of the memory error. The BIOS module 436 may make the physicaladdress available for inspection outside the memory system 400 (e.g., toa user-interface, not shown).

In an example embodiment, the memory controller 422 is substantiallysimilar to the memory controller 222 shown in FIG. 2. The memorycontroller 422 may include a channel interface 413 that is coupled tothe communication channel 416. The communication channel 416 may employa DDR bus that transfers data on both the rising and falling edges of aclock signal for data and a single data rate (SDR) bus that transfersaddress and control on each rising edge of clock.

The mapping module 406 may include an interface 403 that is coupled tothe communication channel 416. The interface 403 may be equipped totransmit and receive data over a DDR bus. The mapping module 406 mayalso include a map 408 that is substantially similar to the map 208 ofFIG. 2. The mapping module 406 may also be coupled to the storage module414.

The storage module 414 may be used to store all or a portion of a memoryaddress that is the result of mapping a memory address. In an exampleembodiment, the storage module 414 may include a data structure (e.g., aLUT 415, an index, a linked list or any other data structure)appropriate to facilitate locating all or a portion of a mapped memoryaddress.

The mapping module 406 may further include interfaces 405, 407, 409 and411 to couple the mapping module to the eight physical RDIMMs 430-437via the four sub-channels 425 through 428 respectively. Each physicalRDIMM 430-437 may include multiple sets of memory devices (e.g. shown inFIG. 6). In an example embodiment, the memory devices are grouped withina larger set of memory devices called a physical rank. A physical rankmay be a set of memory devices. Each physical RDIMM 430-437 may includemultiple physical ranks. A memory address received from the memorycontroller 422 may identify a physical rank located within any one ofthe physical RDIMMs 430-437.

In an example embodiment, the communication channel 416 and thesub-channels 425-428 may include an address bus and a 72 bit wide databus. The physical rank described above may include a set of memorydevices (e.g., DRAMs) that attaches to the full width of a channel(e.g., the communication channel 416 and/or communication sub-channels425-428) and is addressed via identification of a physical rank.

As described above, the RDIMMs 430-437 may include multiple ranks whichin turn may include multiple memory devices. Each memory device mayinclude multiple banks (e.g., see FIG. 6). A bank may be anindependently addressable sub-array within a memory device (for example,a DRAM). Each bank within a DRAM can have an open (e.g., an activated)row, ready to be read, written or pre-charged. In an example embodiment,a read operation (e.g., initiated by the memory controller 422 with aread command) may fetch data from a memory device (e.g., via a column inan open row of a bank). A write operation may store data to a memorydevice (e.g., via a column in an open row of a bank).

In an example embodiment a DRAM includes eight internal banks. Eachindividual DRAM may be four or eight bits wide, meaning either four oreight bits at a time may be transferred from that particular DRAM to thecommunication channel (e.g., communication channel 416 or sub-channels425-428). Since each rank attaches to the full width of the datachannel, the width of a DRAM may determine the number of DRAMs within arank. In an example embodiment, the memory controller 422, thecommunication channel 416 and the sub-channels 425-428 support eightranks of DRAM devices per channel. In an example embodiment, the densestRDIMM for a given DRAM density (e.g., DRAM density may be 512 megabits,1 Gb, 2 Gb, 4 Gb, etc.) is a quad rank (e.g., four ranks) RDIMM, builtusing ×4 devices (e.g., four bits wide). A quad rank ×4 RDIMM with eighterror correction bits has 72 devices (e.g., four ranks ×18 devices perrank).

Logical RDIMMS 418 and 419 may be the RDIMMs that the memory controller422 initially addresses via the communication channel 416. The physicalRDIMMs 430-437 may be the RDIMMs that the memory controller 422 actuallyaddresses after the initial address is mapped with the mapping module406. Logical RDIMMs 418 and 419 may include logical ranks 439-446 thatare the set of all memory devices (for example, DRAMs) that can beaccessed via a single rank ID from the memory controller 422. In anexample embodiment, each logical rank 439-446 is a physical RDIMM (e.g.,one of the physical RDIMMS 430-437) and maps to four physical ranks.

FIG. 5 is a flow diagram of a method 500 for programming a memorycontroller in accordance with an example embodiment. At block 502, themethod 500 may include retrieving first initialization data from aphysical DIMM. As shown in FIG. 4, the BIOS module 436 is coupledbetween the RDIMMs 430-437 and the memory controller 422 and may beconfigured to retrieve the initialization data from the physical RDIMMs.

Examples of the types of information the BIOS module 436 retrieves mayinclude SPD information indicating the capacity of each physical RDIMM,a number of ranks per physical RDIMM, timing information, latencyinformation, a manufacturer of the RDIMM or other information, etc.concerning the physical RDIMMs 430-437.

At block 504, the method 500 may include converting the firstinitialization data to second initialization data for a logical DIMMthat is mapped to the physical DIMM. In an example embodiment, the BIOSmodule 436 may perform the conversion using a linear function (e.g., bymultiplying a first initialization value by a factor to get the secondinitialization value).

At block 506, the method 500 may include programming a memory controllerbased on the second initialization data. In an example embodiment, theBIOS module 436 uses the converted initialization data to program thememory controller 422. It is to be noted that in example embodiments,the memory controller may be programmed based on initialization data fora logical rank of memory devices, when the memory controller is actuallyconnected with physical ranks of memory devices.

FIG. 6 is a chart showing a memory expansion map 600, in accordance withan example embodiment. The memory expansion map 600 maps an addressreceived by the mapping module 406 to sub-channels (e.g., column 604 andsub-channels 425-428) and sub-ranks (e.g., physical ranks) locatedwithin the physical RDIMMs 430-437.

Column 602 generally shows logical rank IDs and two bits (e.g., A15-A14)of multi-bit row addresses. A full multi-bit row address may identify arow of memory cells located within a memory device of a logical rank(e.g., within logical RDIMMs 418-419) as well as within a physical rank(e.g., within physical RDIMMs 430-437).

In an example embodiment, the mapping module 406 extracts the logicalrank ID and the last two bits of a row address from an incoming addressand references the memory expansion map 600 to identify a correspondingsub-channel and sub-rank (e.g., physical rank). In the ninth entry 608of column 602, the “2” represents the rank ID of a logical rank and the“00” are the last two bits of a row address. When an address is mapped,these values map to a particular sub-channel and sub-rank (e.g., aphysical rank). Column 604 represents a corresponding sub-channel, forexample, one of the sub-channels 425-428 of FIG. 4. Column 605represents a corresponding sub-rank (e.g., one of the sub-ranks includedwithin a logical rank of a logical RDIMM).

As described above with respect to FIG. 4, each logical RDIMM 418-419includes four logical ranks. Each logical rank is actually a physicalRDIMM consisting of four physical ranks. Those physical ranks are thesub-ranks of the logical ranks in the RDIMMs 418 and 419.

In the example memory expansion map 600, an address indicating a logicalrank ID of “0” or “1” may be directed by the mapping module 406 tosub-channel “0”. An address indicating a logical rank ID of “2” or “3”may be directed to sub-channel “1” by the mapping module 406 and soforth, etc. As can be seen in the map 600, the first four entriesinclude logical rank IDs of “0”. Thus, the entries all correspond to thesame logical rank (but note that one logical rank e.g., may correspondto the four sub-ranks of column 606 indicated by the values “0,” “1,”“2,” and “3”).

It is to be appreciated that a single sub-channel, as indicated by likenumerals (e.g., 0's, 1's, etc.) in column 604, corresponds to eightsub-ranks. As described above with respect to an example embodiment,eight physical ranks may be supported by a DDR channel.

FIG. 7 is a block diagram showing example relationships 700 between thelogical RDIMMs 706, 704 and the physical RDIMMs 726, 728, 730, 732, 734,736, 738 and 740 respectively. The example RDIMM 706 includes logicalranks 708, 710, 712 and 714. The logical ranks 708, 710, 712 and 714further include the memory devices 716, 718, 720 and 722 respectively.The physical RDIMMS 726, 728, 730, 732, 734, 736, 738 and 740 furtherinclude four each of the physical ranks 741-772 respectively. In anexample embodiment, the logical rank 708 corresponds to the physicalRDIMM 726 which includes the physical ranks 741-744 (e.g., thesub-ranks). There may be similar correspondences between other logicaland physical ranks.

The example address 702 indicates a rank ID of “0” which is a logicalrank, a bank ID of “7” and a row address with its last two bits (e.g.,A15-A14) being “10”. Referring to the map of FIG. 6, a logical rank withthe rank ID of “0” in combination with A15-A14 bits of “10” maps tosub-rank two, (e.g., which is physical rank 743 of the physical RDIMM726) over sub-channel zero. The bank and the remaining row address bits(A13-A0), however, are common to both the logical and physical memorydevices.

A number of banks 724 and 774 corresponding to a memory device withinthe logical rank 708 and the physical rank 743, respectively, are shownwith the rows and columns of a particular bank to illustrate thelocation of memory cells in a row which is indicated by example bank androw number. In an example embodiment, a bank ID of “7” and a row address“ABC01” may be used to identify bank “7” row ABC01 in the logical memorydevice (e.g., banks 724) and the physical device (e.g., banks 774). Thefigure also shows similar relationships between logical ranks 710, 712and 714 and the sub-ranks 745 through 748, 749 through 752 and 753through 756 respectively.

Referring again to FIG. 4, the memory controller 422 may produce anaddress and send the memory address along with an activate command overthe communication channel 416 (e.g., a DDR channel) to the mappingmodule 406. The mapping module 406 may recognize that the memory addressidentifies the location of a logical rank (e.g., logical rank 439) witha logical rank ID. The mapping module 406 may also recognize that thememory address includes a bank ID and row address.

In an example embodiment, the memory address is mapped to a secondlocation of a physical rank where the activate command is asserted. Thephysical rank may be one of the 32 physical ranks (e.g., a plurality ofsets of memory cells) within the physical RDIMMs 430-437. In mapping thememory address, the mapping module 406 may reference the map 408 toassociate the logical rank ID and one or more bits of the row addresswith the physical rank.

FIG. 8 is a flow diagram of a method 800 for storing locationinformation, in accordance with an example embodiment. At block 802 themethod 800 may include receiving an activate command to activate a rowof memory cells. The activate command may be received incidental (e.g.,along with) to the receiving of a memory address (e.g., as describedabove). The activate command received by the mapping module 406 may beto activate a row of memory cells (e.g., the row being indicated by thememory address) within the logical ranks of the logical RDIMMs 418 and419 but may actually activate a row within a physical rank.

At block 804, the method 800 includes initiating a storing of locationinformation based on the receiving of the activate command. In FIG. 4.the mapping module 406 may be coupled to and communicate with thestorage module 414. In an example embodiment, in response to receivingthe activate command the mapping module 406 may store locationinformation within the storage module 414. The location information maybe related to a physical rank selected from the physical ranks 454-485whose location is encoded by the map 408 within the mapping module 406(see FIG. 4).

At block 806, the storing of the location information includes selectinga LUT based on the logical rank ID. As described above, the storagemodule 414 may include a LUT for every logical rank 439-446. In anexample embodiment, the mapping module 406 selects one of the LUTs basedon the logical rank ID received with the memory address.

At block 808 the storing of the location information further includesupdating the LUTs with the one or more bits of the row address indexedby the bank ID. Once a LUT 415 has been selected, the mapping module 406may update the LUT 415 with one or more bits of the row address (e.g.,from the memory address referred to above) indexed with the bank IDreceived with the memory address. If the location information is laterdesired (e.g., to locate an activated row of memory within a physicalrank), the bank ID may be used as an index to the one or more row bits.The logical rank and ID and the one or more row bits may then be used(e.g., in a map) to identify the physical rank 454-485 for which thelocation information was originally stored.

In an example embodiment, the mapping module 406 may detect the receiptof a memory access request that includes logical rank ID and a bank ID.Example memory access requests may include a read command, writecommand, pre-charge command, or any memory access command targeting arow of memory, etc. The location information may be accessed to identifythe target physical rank of the memory access request.

The mapping module 406 may identify an appropriate LUT 415 in thestorage module 414 based on the logical rank ID received with the memoryaccess request. The mapping module 406 may then use the bank ID (e.g.,received with the memory access request), as an index to the one or morebits of the row address. With the logical bank ID and the bits of therow address, the mapping module 406 may determine that the physical rank(e.g., one of the physical ranks 454-485), corresponding to the logicalrank ID and the one or more bits of the memory address, includes anactivated row of memory. In an example embodiment, the mapping module406 may direct the memory access request to the identified activatedrow. If the memory access command is a read or write command, then theactivated row is to be read or written, respectively. If the memoryaccess command is a pre-charge command, the activated row is to bepre-charged, and so on.

In some example embodiments, the memory controller 422 may assertconsecutive write commands to the same logical rank (e.g., one of thelogical ranks 439-446 in the logical RDIMM 418) but the commands may bemapped to different physical ranks on the same physical RDIMM (e.g.,physical ranks 454-457 within the physical RDIMM 430). In an exampleembodiment, a physical rank being written provides termination during awrite operation. Termination may be used to regulate a level ofelectrical noise on a communication channel. To avoid doubly terminatingthe communication sub-channel (e.g., a bus) when the signal at the endof one write transfer overlaps the preamble signal of a subsequentwrite, the memory controller 422 may introduce a bus cycle between thewrite operations during which no data is transferred.

Alternatively or additionally, consecutive write operations may besupported without introducing a bus cycle where no data is transferred.For example, one physical rank (e.g., physical rank 454) may providetermination for all writes to the physical RDIMM 430, while anotherphysical rank (e.g., physical rank 455) provides termination for anelectrical stub that the RDIMM (e.g., RDIMM 430) becomes when a write orread command targets another physical RDIMM (e.g., physical RDIMM 431).

It is to be appreciated that a physical memory device may return anincorrect sequence of bits in response to a memory request. In anexample embodiment, RDIMMs 430-437 include one or more memory devices tostore ECC bits to allow the memory controller 422 to detect and possiblycorrect memory errors.

FIG. 9 is a flow diagram illustrating a method 900 for reporting amemory error, in accordance with an example embodiment. At block 902,the method 900 may include receiving a logical memory address associatedwith a memory error. The BIOS module 436 is shown to be coupled with thememory controller 422 and may receive the logical memory addressassociated with the memory error from the memory controller 422.

In an example embodiment, the memory controller 422 may detect thememory error using an 8 bit ECC code. The error detected by the memorycontroller 422 may be a one or two bit error and the memory controller422 may correct those errors that are one bit errors. In some exampleembodiments, multiple bit errors may be both detected and corrected bythe memory controller 422.

Block 904 of the method 900 may include mapping the logical memoryaddress to a physical memory address. In an example embodiment, the BIOSmodule 436 may reference the map module 440 that maps the logical memoryaddress of the memory error to a physical memory address on a physicalmemory device (e.g., the physical RDIMM 430).

At block 906, the method 900 may conclude with indicating the physicalmemory address as a location of the memory error. In various exampleembodiments, the BIOS module 436 may indicate the physical memoryaddress as a physical location of a memory error. The BIOS module 436may indicate the physical memory address as a physical location of amemory error by signaling a user interface (e.g., a flashing lightemitting diode, a warning message on a computer display or an indicationusing any appropriate user interface, etc.).

FIG. 10 is a block diagram of a memory expansion system 1000, inaccordance with an example embodiment. In general, the memory expansionsystem 1000 shows an example embodiment of a system to distribute data,address information and control information as the data flows between asingle channel 1001 and multiple sub-channels and sub-ranks (e.g.,physical ranks or sub-ranks 1017 of the physical RDIMMs 1016 andsub-channels 1003, 1005, 1007, 1009).

The mapping module 1002 is shown to couple with four communicationsub-channels over the data paths 1010, 1012 and 1014. The data paths1010, 1012 and 1014 are shown to couple with the physical RDIMMs 1016.The physical RDIMMs 1016 may include the 32 sub-ranks 1017. Mapping tothe sub-ranks 1017 may be performed in a substantially similar fashionto what has been described above. The sub-channels 1003, 1005, 1007 and1009 coupled to the physical RDIMMs 1016 may be 72 bits wide, and have64 data and eight ECC bits. In an example embodiment, the physicalRDIMMs 1016 are quad-ranked and use x4 devices.

In an example embodiment, the mapping module 1002 includes electroniccircuitry 1004, 1006, 1008 (e.g., a chipset) to map the DDR3 channel1001 to the four sub-channels 1003, 1005, 1007 and 1009. The address andcontrol path circuitry 1004 may include a map and control unit 1011(discussed in more detail with respect to FIG. 12) to map addresses toappropriate sub-ranks and sub-channels and to manage memory commands.The address and control path circuitry 1004 may also include a data pathunit 1013 (discussed in more detail with respect to FIG. 15) toconfigure the data connection circuitry 1006, 1008. The address andcontrol path circuitry 1004 may communicate control signals to the dataconnection circuitry 1006, 1008 via paths 1018 and may communicate clocksignals to the data connection circuitry 1006 via the paths 1020. Dataconnection circuitry 1006, 1008 connects the data and strobe lines fromthe host DDR3 channel 1001 onto the four sub-channels 1003, 1005, 1007and 1009. Example data connection circuitry 1006, 1008 may useFET-switched paths between the data channel at the memory controller tothe data sub-channels at the physical RDIMMs 1016. A FET switch is adevice that has a low “on” resistance between a source and a drain, ahigh “off” resistance and a low capacitance. In an example embodiment, aFET switch may approximate the behavior of a wire when “on” and an opencircuit when “off”.

FIG. 11 is a flow diagram of a further method 1100 for memory expansion,in accordance with an example embodiment. At block 1102, the method 1100includes receiving a memory request from a memory controller over afirst channel. In an example embodiment, the map and control unit 1011is configured to receive the memory request from a memory controller(not shown) over the DDR3 channel 1001.

At block 1104, the method 1100 includes reacting to the memory requestby selecting a location in memory that is to be connected to asub-channel. The memory controller may direct the memory request to alogical rank and based on the memory request, the map and control unit1011 may select a location within memory (e.g., map the memory requestto one of the sub-ranks 1017 located on one of the physical RDIMMs 1016)to connect with one of the sub-channels 1003, 1005, 1007, 1009.

Block 1106 of the method 1100 may include configuring a set of FETs toconnect the first channel with the sub-channel while block 1108 mayinclude allowing data to flow between the memory controller and thelocation in the memory (e.g., one of the sub-ranks 1017) over the firstchannel and the selected sub-channel 1003, 1005, 1007, 1009. In anexample embodiment, the memory request may include a column address anda read or a write command. Once the sub-rank to read or write has beendetermined by the map 1011 (e.g., mapped from the logical rank targetedby the memory request), the data path unit 1013 may cause the FETs toconnect the DDR3 channel 1001 with the designated sub-channel 1003,1005, 1007, 1009 so that the read or write data can flow to anintersection of the column address and an activated row in thedesignated sub-rank 1017.

The data path unit 1013 may update the data path circuitry 1006, 1008 toconnect with different sub-channels 1003, 1005, 1007, 1009 depending onthe memory request (and e.g., logical rank indicated by the memoryrequest) to allow data to flow between the memory controller and otherof the sub-ranks 1003, 1005, 1007, 1009.

During initialization of the memory system, the memory controller (notshown) may perform read training and/or write leveling for each logicalrank to align data signals with clock signals. Sub-ranks 1017 mapped tothe logical rank and co-located on one of the RDIMMs may experience verysimilar voltages, temperatures, and timing conditions. In exampleembodiments where one of the physical RDIMMs 1016 maps to the logicalrank, four of the sub-ranks 1017 residing on a physical RDIMM 1016 mayadopt the same read training and write leveling values that wereassigned to the logical rank. Thus, in various example embodiments, datamay flow through the data connection circuitry 1006 and 1008 withoutbeing retimed to align with a clock signal on the mapping module 1002.

FIG. 12 shows a block diagram of example address and control pathcircuitry 1202 that includes a map and control unit 1204, in accordancewith an example embodiment. The output 1216 of the map and control unit1204 includes chip select numbers (e.g., the physical rank ID) andseparate command bits for each logical rank.

In an example embodiment, the number of instances of the map and controlunit 1204 is equal to the number of logical ranks as seen by the memorycontroller (not shown). The example map and control unit 1204 mayinclude an address decoder 1208, a command decoder 1210, a serializerstate machine 1214, a sub-rank LUT 1212 and various other supportinglogic components.

In FIG. 12, address and command bits 1206 are received by the map andcontrol unit 1204 from a memory controller. The chip select number(e.g., the logical rank ID) and the last two bits of the row address(e.g., notated as A[15:14] for logical RDIMMs based on 4 Gbit DRAMs) arereceived by the address decoder 1208 that may decode the two bits to mapthe chip select number 1206 to one of the four physical ranks (e.g.,with a 2:4 decoder with enable). The address decoder 1208 may referencea map such as the map in FIG. 6 to provide the corresponding physicalsub-ranks and sub-channels.

The command decoder 1210 may decode various commands and forward them toother logic components depending on the command. When a read or writecommand is received by the command decoder 1210, the LUT 1212 may beused to determine the physical rank in which a row of memory should beread or written. The example LUT 1212 for a particular logical rank mayaccomplish this by associating a bank address with a particular physicalrank when a most recently received activate command activates a row inthat physical rank. When a read, write or pre-charge command isreceived, the LUT 1212 may use the bank address specified in the read,write or pre-charge command as an index into the LUT 1212 to obtain thetarget physical rank. The association between bank addresses and thephysical ranks may allow the LUT 1212 to locate rows of memory activatedfor subsequent read, write, pre-charge or any other commands, etc.

The serializer state machine 1214 may receive memory commands associatedwith a logical rank from the command decoder 1210, and if necessary,provide multiple copies of the memory command for physical ranks mappedto the logical rank (discussed in further detail below).

Various memory commands may be received by the serializer state machine1214 in example embodiments. A refresh command is a DRAM command thatreads and then restores the contents of one or more rows of memory toprevent data loss due to charge leakage or other effects. The ZQCL andZQCS calibration commands may trigger calibration cycles in all physicalranks mapped to a logical rank. A ZQCL command is a DRAM command issuedonce per rank at boot time that triggers the initial calibration ofdriver output impedances and on-die termination impedances. The ZQCScommand is a DRAM command issued periodically to track slow variationsin voltage and temperature and triggers an update to driver outputimpedance and on-die termination impedance.

Other commands, such as pre-charge all and mode register write commands,may similarly be serialized by the serializer state machine 1214. Thepre-charge command is a DRAM command that writes the contents of allopen row/banks back to the array and then closes all row/banks. The moderegister command is a DRAM command that may write DRAM latency values orother configuration information to a mode register located on a memorydevice. The example DDR3 DRAMs may contain mode registers MR0, MR1, MR2and MR3. MRS commands are typically issued only by the BIOS duringsystem initialization.

FIG. 13 is a flow diagram illustrating a method 1300 for distributing amemory command, in accordance with an example embodiment. Block 1302 ofmethod 1300 may include detecting a memory command directed to a logicalrank. As shown in FIG. 12, the command decoder 1210 receives commandbits 1206 and may detect memory commands directed to a logical rank.

Block 1304 may include detecting a number of physical ranks mapped tothe logical rank. The address decoder 1208 shown in FIG. 12 may mapphysical ranks to the logical rank. In an example embodiment, theaddress decoder 1208 is to map the number of physical ranks to thelogical rank based on a number of sets of DRAMs known to provide a fulldata-bit width of an RDIMM to be driven on a data bus.

Block 1306 of method 1300 may include determining whether the memorycommand is to be issued to the number of physical ranks. The serializerstate machine 1214 may determine that a particular memory command is tobe issued to mapped physical ranks if the serializer state machine 1214detects the command as a refresh command, a ZQCL or ZQCS calibrationcommand, a pre-charge-all command, a mode register write command, or anyother command that should be distributed to a mapped physical rank, etc.

In example embodiments, an activate command to open a logical page (ore.g., row) of a certain size (e.g., 2K Bytes) is received by the map andcontrol unit 1204. The memory controller that sends the command willexpect the logical page to be available for subsequent reads and writes.If the example address decoder 1208 determines that the logical page islarger in size than the size of the physical page (e.g., 1K Bytes) towhich the logical page is mapped (e.g. two times as large) then multiplephysical ranks are to be issued the activate command to support thesubsequent reads and writes to the logical page.

Block 1308 may include issuing the memory command to the physical ranks(e.g., the physical ranks mapped to the logical rank) based on adetermination that it is appropriate to issue the memory command to thenumber of physical ranks. In FIG. 12, the serializer state machine 1214is to receive the decoded command from command decoder 1210 and if theserializer state machine 1214 has determined it to be appropriate (e.g.,as described with respect to block 306), issue the memory command to thenumber of physical ranks mapped to the logical rank by the addressdecoder 1208.

In an example embodiment, the number of copies of the memory commandissued by the serializer state machine 1214 depends on the mappingconfiguration being used. If each logical rank maps to four physicalranks on a physical RDIMM, then four copies are to be sent for certainmemory commands. If each logical rank maps to two physical ranks on aphysical RDIMM, then two copies are to be sent for certain memorycommands.

In some example embodiments, the serializer state machine 1214 is toissue the selected memory command over multiple command cycles. Thenumber of clock cycles may depend on the configuration of the RDIMMsbeing used as memory. For example, RDIMMs configured for four ranks mayaccept up to two commands per cycle while those configured for one andtwo ranks may accept at most one command per cycle.

In an example embodiment, a refresh command to refresh cells within alogical rank may trigger the refresh of memory cells within physicalranks mapped to the logical rank. The four physical ranks mapped to thelogical rank may share a common command bus (and e.g., a commonsub-channel) and the serializer state machine 1214 may be triggered toissue a refresh command to the four physical ranks on two consecutiveclock cycles. In an example embodiment, refresh commands mapped tosub-channel “0” and physical ranks “0” and “2” are (see e.g., FIG. 6)sent in the first cycle, and the refresh commands for physical ranks “1”and “3” are sent in the second cycle.

In the example case of an activate command directed to a logical page, 8Gbit (2 Gb×4) DRAMs are emulated with 4 ranks of 2 Gbit (512 Mb×4) DRAMsand the sizes may be characterized as follows:

DRAM DENSITY PAGE SIZE ROW BITS COLUMN BITS 8 Gbit (2 Gb × 4) 2 KBytesA[15:0] A[13, 11, 9:0] 2 Gbit (512 Mb × 4) 1 KBytes A[14:0] A[11, 9:0]

As described above, the address decoder may determine that an activatecommand is to be forwarded to two physical ranks because the logicalpage size is twice as large as the physical page size. In a case whenthe example address decoder detects that A[15]=0, the address decodermay cause the physical ranks 0 and 2 to be activated and cause thesequence 1010 to be stored in a LUT entry indexed by BA[2:0]. In a casewhen A[15]=1, the physical ranks 1 and 3 may be activated and 0101stored in the LUT 1212 entry indexed by BA[2:0]. On a read or writecommand, the example address decoder 1208 examines A[13] along with theLUT 1212 entry to determine the physical ranks to read or write. Theexample correspondence would appear in a table such as the as following:

A[13] LUT[BA] CS[3:0] 0 1010 1110 0 0101 1101 1 1010 1011 1 0101 0111

In an example case where an activate command for a logical page isissued to multiple physical ranks to open a logical page, a prechargecommand is issued to the same multiple physical ranks to close thelogical page.

As explained above, the address and control path circuitry 1002 mayprovide the map and control unit 1204 and data path unit 1504 to map 4Gb based logical ranks to four 1 Gb-based physical ranks over asub-channel. In an example embodiment, the processing by the address andcontrol path circuitry 1002, 1502 may add a cycle of latency to thememory expansion system 1000.

FIG. 14 is a flow diagram illustrating a method 1400 for modifying aninitialization value to account for added memory system latency, inaccordance with an example embodiment. At block 1402, the method 1400may include detecting a mode register write command (e.g., with thecommand decoder 1210) associated with a logical rank while block 1404may include receiving a data value (e.g., at a modification module 1220)associated with the mode register write command.

Block 1406 may include modifying the data value based on the detectingof the mode register command and at block 1408, the method 1400 mayconclude with forwarding the modified data value to a number of physicalranks (e.g., physical ranks mapped to the logical rank).

In an example embodiment, the modification module 1220 receives a datavalue associated with the mode register write command. Upon receivingthe mode register write command for a logical rank, the serializer statemachine 1214 may be further configured to signal a multiplexer 1218 thatreceives input from the modification module 1220. The modificationmodule 1220 may modify the data value before a modified data value isforwarded to physical ranks mapped from the logical rank. In an exampleembodiment, the modification module 1220 is to decrement mode registerdata values to account for the cycle of latency added by the address andcontrol path circuitry 1202, 1502.

FIG. 15 is a block diagram illustrating example address and control pathcircuitry 1502 that includes a data path unit 1504, in accordance withan example embodiment. Also shown is data connection circuitry 1510configured to use FETs 1512 to distribute data on the data paths 1012and 1014. The example data path unit 1504 may include a command decoder1505 that may receive the same memory commands, 1503 as received 1206 bythe map and control unit 1204 of FIG. 12.

FIG. 16 is a flow diagram illustrating a method 1600 for controlling adata flow, in accordance with an example embodiment. At block 1602, themethod begins with receiving a memory request from a memory controller.In FIG. 15, the command decoder 1505 may decode a memory command andforward it to be associated with a logical rank chip select shown to bereceived by the data path unit 1504.

At block 1604, read or write latency associated with the memory requestis to be determined so that at block 1606 the determined read or writelatency may be used to control data flow timing (e.g., data flowingbetween the memory controller and physical memory). Referring again toFIG. 15, the data path unit 1504 may receive chip select information atthe chip select input 1501. The data path unit 1504 may then pair adecoded memory request with corresponding chip select information. Theappropriate timing controller 1507, 1509, 1511, 1513, 1515, 1517, 1519or 1521 may then provide stored latency information (e.g., column accessstrobe (CAS) latency and additive latency (AL)) associated with thedecoded command and apply the appropriate time controls.

At block 1608, the method may include allowing the data to flow betweenthe memory controller and the location in memory (e.g., over the firstchannel and the sub-channel). In an example embodiment, the FET controlsignals may be encoded (e.g., by the encoders 1506) and forwarded to thedata connection circuitry 1510 where the FET switches 1512 may beupdated to allow data to pass between the memory controller and thephysical RDIMMs. As described above, the FET switches 1512 may be usedto steer the data to the appropriate sub-channel.

In some embodiments, a pipelined data path may be employed instead of aFET based data path. The pipelined data path may provide re-timing ofboth read and data transfers, and may include data bus termination, readleveling, write leveling, read strobe centering, and write strobecentering.

With further regard to latency, timing of read bursts and write burstsrelative to read and write commands is determined by the CAS latency andadditive latency. The address and control path circuitry 1202, 1502 mayprovide a data path between the memory controller and physical memory ina timely fashion such that the memory devices may be accessed while thememory expansion configuration can remain transparent to the memorycontroller. Example timing may include the following:

A read burst begins at a time after a read latency (RL) determined by:RL=AL+CL cycles after a read command

The read preamble may occupy one cycle prior to the read burst. In anexample embodiment, the data path may permit the proper FET switch pathsto connect the host channel to the communication sub-channel beingaccessed at a number of cycles after the read command determined by:RL−1=AL+CL−1

The connection between the host channel and communication sub-channelmay be maintained until the next read or write command causes a newconnection to be made.

A write burst begins at a time after a write latency (WL) determined by:WL=AL+CWL

The write preamble occupies one cycle prior to the write burst. In anexample embodiment, the data path may permit the proper FET switch pathsto connect the host channel to the sub-channel at a number of cyclesafter the write command determined by:WL−1=AL+CWL−1

The connection between the host channel and the communicationsub-channel may be maintained until the next read or write commandcauses a new connection to be made.

The above description is intended to be illustrative and notrestrictive. For example, one or more aspects of the above-describedembodiments may be used in combination with each other. Otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the claims should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled. In the appendedclaims, the terms “including” and “in which” are used as theplain-English equivalents of the respective terms “comprising” and“wherein.” Also, in the following claims, the terms “including” and“comprising” are open-ended, that is, a system, device, article, orprocess that includes elements in addition to those listed after such aterm in a claim are still deemed to fall within the scope of that claim.Moreover, in the following claims, the terms “first,” “second,” and“third,” etc. are used merely as labels and are not intended to imposenumerical requirements on their objects.

The Abstract is provided to comply with 37 C.F.R. §1.72(b), whichrequires that it allow the reader to quickly ascertain the nature of thetechnical disclosure. It is submitted with the understanding that itwill not be used to interpret or limit the scope or meaning of theclaims. Also, in the above Detailed Description, various features may begrouped together to streamline the disclosure. This should not beinterpreted as intending that an unclaimed disclosed feature isessential to any claim. Rather, inventive subject matter may lie in lessthan all features of a particular disclosed embodiment. Thus, thefollowing claims are hereby incorporated into the Detailed Description,with each claim standing on its own as a separate embodiment.

What is claimed is:
 1. A method comprising: detecting a memory command directed to a logical rank; detecting a number of physical ranks mapped to the logical rank from a plurality of physical ranks; determining whether the memory command is to be issued to the number of physical ranks; and issuing the memory command to the number of physical ranks based on a determination that the memory command is to be issued to the number of physical ranks, wherein issuing the memory command includes using a look-up table with the logical rank and a portion of a logical row address of the logical rank to identify each of the physical ranks mapped to the logical rank and a corresponding sub-channel for transmitting physical-memory-element values between a memory controller and said each of the physical ranks mapped to the logical rank, the corresponding sub-channel being selected from a plurality of sub-channels that transmit data between the memory controller and the physical ranks, the corresponding sub-channel being configured to transmit data between the memory controller and said each of the physical ranks mapped to the logical rank by enabling data transmission in the corresponding sub-channel in response to the identification of the corresponding sub-channel, and the physical-memory-element values being stored at one or more memory locations of said each of the physical ranks mapped to the logical ranks.
 2. The method of claim 1, wherein the detecting of the memory command includes the detecting of a activate command to activate a first row of logical memory in the logical rank, and the determining of whether the activate command is to be issued to the number of physical ranks includes determining whether the first row of logical memory is a same size as a second row of physical memory in one of the physical ranks mapped to the logical rank.
 3. The method of claim 1, wherein the detecting of the memory command includes the detecting of a mode register write command, and the method further comprises: receiving a data value associated with the mode register write command; modifying the data value based on the detecting of the mode register write command to account for latency differences between logical memory devices included in the logical rank and physical memory devices included in the number of physical ranks; and forwarding a modified data value to the number of physical ranks.
 4. The method of claim 1, wherein the detecting of the number of physical ranks mapped to the logical rank includes detecting a number of sets of dynamic random access memory devices on a dual inline memory module (DIMM), wherein each set is configured to provide a full data-bit width of the DIMM to be driven on a bus.
 5. The method of claim 1, wherein the issuing of the memory command to the number of physical ranks includes issuing the memory command to the number of physical ranks over a plurality of command cycles when a plurality of the physical ranks share a common command bus, each physical rank being issued the command in a corresponding one of the command cycles.
 6. The method of claim 1, wherein the determining that the memory command is to be issued to the number of physical ranks is based on detecting at least one of a refresh command, an initial calibration of impedance command, a periodic calibration of impendence command, a pre-charge-all command and a mode register write command.
 7. A system comprising: decoding means for detecting a memory command directed to a logical rank; mapping means for detecting a number of physical ranks mapped to the logical rank from a plurality of physical ranks; and issuing means for determining whether the memory command is to be issued to the number of physical ranks and for issuing the memory command to the number of physical ranks based on a determination that the memory command is to be issued to the number of physical ranks, wherein issuing the memory command includes using a look-up table with the logical rank and a portion of a logical row address of the logical rank to identify each of the physical ranks mapped to the logical rank and a corresponding sub-channel for transmitting physical-memory-element values between a memory controller and said each of the physical ranks mapped to the logical rank, the corresponding sub-channel being selected from a plurality of sub-channels that transmit data between the memory controller and the physical ranks, the corresponding sub-channel being configured to transmit data between the memory controller and said each of the physical ranks mapped to the logical rank by enabling data transmission in the corresponding sub-channel in response to the identification of the corresponding sub-channel, and the physical-memory-element values being stored at one or more memory locations of said each of the physical ranks mapped to the logical ranks.
 8. The system of claim 7, wherein the decoding means is to detect a mode register write command, and the system further comprises: modifying means for receiving a data value associated with the mode register write command and for modifying the data value to account for latency differences between logical memory devices included in the logical rank and physical memory devices included in the number of physical ranks before forwarding a modified data value to the number of physical ranks.
 9. The system of claim 7, wherein the mapping means is for mapping the number of physical ranks to the logical rank based on a detection of a number of sets of dynamic access devices on a dual inline memory module (MINIM), wherein each set is configured to provide a full data-bit width of the DIMM to be driven on a data bus.
 10. The system of claim 7, wherein the issuing means is for issuing the memory command to the number of physical ranks over a plurality of command cycles when a plurality of the physical ranks share a common command bus, each physical rank being issued the command in a corresponding one of the command cycles.
 11. The system of claim 7, wherein the issuing means is for determining that the memory command is to be issued to the number of physical ranks based on whether the issuing means detects at least one of a refresh command, an initial calibration of impedance command, a periodic calibration of impedance command, a pre-charge-all command and a mode register write command.
 12. A system comprising: a command decoder to detect a memory command directed to a logical rank; a mapping module to map physical ranks to the logical rank from a plurality of physical ranks; and a serializer communicatively coupled to the command decoder and configured to issue the memory command to a number of physical ranks mapped to the logical rank by the mapping module, based on a determination that the memory command is to be issued to the number of physical ranks mapped to the logical rank, wherein the serializer is configured to issue the memory command by using a look-up table with the logical rank and a portion of a logical row address of the logical rank to identify each of the physical ranks mapped to the logical rank and a corresponding sub-channel for transmitting physical-memory-element values between a memory controller and said each of the physical ranks mapped to the logical rank, the corresponding sub-channel being selected from a plurality of sub-channels that transmit data between the memory controller and the physical ranks, the corresponding sub-channel being configured to transmit data between the memory controller and said each of the physical ranks mapped to the logical rank by enabling data transmission in the corresponding sub-channel in response to the identification of the corresponding sub-channel, and the physical-memory-element values being stored at one or more memory locations of said each of the physical ranks mapped to the logical ranks.
 13. The system of claim 12, wherein the command decoder is to detect a mode register write command, and the system further comprises: a modification module to receive a data value associated with the mode register write command and to modify the data value to account for latency differences between logical memory devices included in the logical rank and physical memory devices included in the number of physical ranks before forwarding a modified data value to the number of physical ranks.
 14. The system of claim 12, wherein the mapping module is to map the number of physical ranks to the logical rank based on a detection of a number of sets of dynamic access devices on a dual inline memory module (DIMM), wherein each set is configured to provide a full data-bit width of the DIMM to be driven on a data bus.
 15. The system of claim 12, wherein the serializer is to issue the memory command to the number of physical ranks over a plurality of command cycles when a plurality of the physical ranks share a common command bus, each physical rank being issued the command in a corresponding one of the command cycles.
 16. The system of claim 12, wherein the serializer is to determine that the memory command is to be issued to the number of physical ranks based on whether the serializer detects at least one of a refresh command, an initial calibration of impedance command, a periodic calibration of impedance command, a pre-charge-all command and a mode register write command.
 17. A non-transitory machine-readable medium having instructions which, when executed by a processing system, cause the processing system to: detect a memory command directed to a logical rank; detect a number of physical ranks mapped to the logical rank from a plurality of physical ranks; determine whether the memory command is to be issued to the number of physical ranks; and issue the memory command to the number of physical ranks based on a determination that the memory command is to be issued to the number of physical ranks, wherein issuing the memory command includes using a look-up table with the logical rank and a portion of a logical row address of the logical rank to identify each of the physical ranks mapped to the logical rank and a corresponding sub-channel for transmitting physical-memory-element values between a memory controller and said each of the physical ranks mapped to the logical rank, the corresponding sub-channel being selected from a plurality of sub-channels that transmit data between the memory controller and the physical ranks, the corresponding sub-channel being configured to transmit data between the memory controller and said each of the physical ranks mapped to the logical rank by enabling data transmission in the corresponding sub-channel in response to the identification of the corresponding sub-channel, and the physical-memory-element values being stored at one or more memory locations of said each of the physical ranks mapped to the logical ranks.
 18. The machine-readable medium of claim 17, wherein a detecting of the memory command includes the detecting of a activate command to activate a first row of logical memory in the logical rank, and a determining of whether the activate command is to be issued to the number of physical ranks includes determining whether the first row of logical memory is a same size as a second row of physical memory in one of the physical ranks mapped to the logical rank.
 19. The machine-readable medium of claim 17, wherein the detecting of the memory command includes the detecting of a mode register write command, and the method further comprises: receiving a data value associated the mode register write command; modifying the data value based on the detecting of the mode register write command to account for latency differences between logical memory devices included in the logical rank and physical memory devices included in the number of physical ranks; and forwarding a modified data value to the number of physical ranks.
 20. The machine-readable medium of claim 17, wherein the detecting of the number of physical ranks mapped to the logical rank includes detecting a number of sets of dynamic random access memory devices on a dual inline memory module (DIMM), wherein each set is configured to provide a full data-bit width of the DIMM to be driven on a bus.
 21. The machine-readable medium of claim 17, wherein the issuing of the memory command to the number of physical ranks includes issuing the memory command to the number of physical ranks over a plurality of command cycles when a plurality of the physical ranks share a common command bus, each physical rank being issued the command in a corresponding one of the command cycles.
 22. The machine-readable medium of claim 17, wherein the determining that the memory command is to be issued to the number of physical ranks is based on detecting at least one of a refresh command, an initial calibration of impedance command, a periodic calibration of impendence command, a pre-charge-all command and a mode register write command.
 23. The method of claim 1, wherein the portion of the logical row address is a first portion and issuing the memory command includes using a second portion of the logical row address as a common address index for the logical rank and said each of the physical ranks mapped to the logical rank.
 24. The method of claim 1, further comprising: in response to identifying the corresponding sub-channel, configuring a plurality of hardware elements to connect a channel for the physical-memory element values to the corresponding sub-channel, the channel being connected to the memory controller, which detects the memory command, the corresponding sub-channel being connected to the one or more memory locations of said each of the physical ranks mapped to the logical rank where the physical-memory-element values are stored, and the physical-memory element values flowing between the memory controller and the one or more memory locations of said each of the physical ranks mapped to the logical rank via the channel and the corresponding sub-channel.
 25. The method of claim 1 wherein issuing the memory command includes accessing the look-up table from a computer-readable medium that stores the look-up table, the look-up table relating a logical rank identifier and the portion of the logical row address to said each of the physical ranks mapped to the logical rank and to the corresponding sub-channel.
 26. The method of claim 1, further comprising: transmitting the physical-memory-element values corresponding to the memory command along the corresponding sub-channel selected from a plurality of sub-channels, the corresponding sub-channel being connected to the one or more memory locations of said each of the physical ranks mapped to the logical rank where the physical-memory-element values are stored. 